Biochemical studies with specially prepared cDNA probes from the Kirsten, Harvey and Moloney strains of murine sarcoma virus have indicated that each strain arose by genetic recombination between a helper type-C virus and sarcoma-specific information endogenous to the species of origin of the virus. This sarcoma-specific information represents in the case of the Kirsten and Harvey virus a unique new class of endogenous mammalian type-C virus that is normally defective for replication but which nevertheless can still transform cells. The identification of such sequences opens new approaches to the identification of the viral cause of neoplasia.